Centrifugal SHS metallurgy of Nb–Ta–Hf-based high-entropy alloys alloyed with Cr, V, Zr, Ti, Ce + La, and Gd, and fabrication of oxide-fiber-reinforced composites with these alloys

High-entropy alloys (HEAs) have attracted considerable research interest. Refractory high-entropy alloys (RHEAs), composed primarily of high-melting-point elements (Nb, Mo, Ta, W, Re, Hf, etc.), are of particular interest. These alloys are considered promising for aviation and aerospace applications at temperatures above 1100 °C. The principal disadvantages of refractory-element-based RHEAs are their very high density (more than 10 g/cm³), poor oxidation resistance, and relatively low creep strength, which limit their suitability for these applications. Oxide-fiber reinforcement is a promising approach to improving these properties. In this study, for the first time, high-temperature composites combining eutectic Al₂O₃–Al₅Y₃O₁₂ oxide fibers with Nb–Ta–Hf-based RHEAs containing transition metals (Cr, V, Ti, and Zr) were produced and investigated. The integrated process comprised the synthesis of novel Nb–Ta–Hf-based alloys containing Cr, V, Zr, and Ti by centrifugal SHS metallurgy, followed by hot pressing to fabricate prototype composites with aligned fibers at volume fractions of 0–40 vol. %. The synthesized RHEAs were subjected to oxidation annealing, and the composites with different fiber contents were tested for high-temperature flexural strength by three-point bending. The RHEAs were synthesized by centrifugal SHS metallurgy, a branch of self-propagating high-temperature synthesis (SHS), using thermite-type SHS mixtures containing powdered oxides of the target elements. The main phase in the synthesized alloys was a body-centered cubic (BCC) solid solution, whereas the  overall phase composition depended on the other alloying elements in the starting mixture. Alloying with Group 4 elements (Ti, Zr, Hf), Group 5 (V), Group 6 (Cr), and rare-earth elements (Ce + La and Gd) promoted the formation of solid-solution phases with different crystal structures. Oxidation-resistance tests identified composition S2-3 (Nb₄₅Ta₂₀Hf₁₅Ti₁₅Cr₅) as the most promi­sing for further study. Compared with compositions reported by other authors, it exhibited good oxidation resistance, making it competitive for high-temperature applications. At 1300 °C, increasing the fiber content from 0 to 25 vol. % increased the three-point flexural strength by a factor of 4.5 to 163 MPa.

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